1. Field of the Invention
The present invention relates to a composition for a polysulfide-based resin which has a high refractive index and which has extremely low dispersion properties, is excellent in heat resistance, impact resistance and scratch resistance, and emits far less sulfur odor at a monomer handling step and at a subsequent working step. Furthermore, the present invention also relates to the polysulfide-based resin, and an optical material, particularly a lens comprising the resin.
2. Description of the Related Art
Plastic lenses are more lightweight, less breakable and more easily dyeable as compared with inorganic lenses, and for this reason, they have rapidly spread in the fields of optical elements such as the lenses of spectacles and cameras.
As conventional resins which have been widely used for these purposes, there are products obtained by the radical polymerization of diethylene glycol bis(allyl carbonate) (hereinafter referred to as "DAC"). These resins are characterized by being lightweight and being excellent in impact resistance, dyeability and working properties such as cutting properties and polishing properties.
However, the refractive index of the DAC lens is n.sub.D =1.50, which is lower as compared with the organic lens (n.sub.D =1.52), and in order to obtain optical characteristics equal to those of the organic lens, it is necessary to increase the center thickness, the edge thickness and the curvature of the lens, and when such requirements are met, the lens is unavoidably thickened on the whole. Therefore, a lens having a higher refractive index has been desired.
The present inventors have already found 1,2-bis(2-mercaptoethylthio)-3-mercaptopropane (hereinafter referred to as "GST") and have also found that a resin having a high refractive index which emits far less sulfur odor at a handling step and at a working step can be obtained by reacting this GST with a compound having two or more reactive unsaturated bonds in one molecule (Japanese Patent Application Laid-open No. 84031/1991). The resin using this GST has the excellent optical characteristics, but the heat resistance of the resin is not sufficient. For example, this kind of resin has a problem that the deformation of its lens easily occurs at a post-step such as dyeing in which heat at about 60 to 90.degree. C. is usually applied to the lens. In addition, depending upon its use purpose, the resin is also insufficient in impact resistance and has no practical scratch resistance.
Furthermore, in Japanese Patent Application Laid-open No. 26089/1995, there have been disclosed a polymer composition for casting which comprises a polythiol containing GST and an aromatic olefin monomer such as divinylbenzene, and a lens comprising the polymer. However, this kind of lens is poor in impact resistance, and what is worse, it does not have practical scratch resistance, either.
Additionally, in Japanese Patent Application Laid-open No. 25240/1993, a composition for lenses has been suggested which comprises a polymer of a polyisocyanate compound, a polythiol compound and a polyene compound. However, in this publication, it has been described that the amount of the polyene compound to be used is preferably 50% by weight or more, and if the polyene compound is used in an decreased amount, sufficient heat resistance and surface hardness cannot be obtained. In addition, when the polyene compound is used in an amount within a use range described herein, the impact resistance of the formed lens having a thickness of 1.5 mm is at most 36 g according to an iron ball dropping test from a height of 127 cm, and this value does not mean that the lens has the sufficient impact resistance. Besides, the obtained lens is breakable and does not possess practical scratch resistance.